1. Field of the Invention
This invention relates to fault detection in a packet switched network and more particularly to the detection and localization of interconnect faults in a packet switched network which uses a status message-based link control protocol.
2. Description of the Related Art
An efficient packet switched network requires dependable electrical connections between nodes in the network. Network errors (faults) can occur if the conductors (for example, a wire) physically linking the nodes in a network are in either a "stuck-at" or "open" state. A conductor is in a "stuck-at" state if the coupling link is erroneously pulled or pushed to a specific voltage level. Thus, the conductor is stuck at the pulled or pushed level regardless of the level of the signal applied to the physical link. An "open" state means that the link is not coupled to the signal but instead carries no signal and thus "floats" at an indeterminate voltage level. Both the "stuck-at" and "open" conditions in an electrical link cause fault conditions in a packet switched network. Fault detection and localization is important in assisting a network manager in network maintenance and during operation, fault detection and localization allows source nodes to re-route packets in order to avoid areas of a network having an unacceptable number of faults. Thus, fault detection and localization is important in maintaining a reliable packet switched network. One conventional method of fault detection uses error codes, such as a cyclic redundancy code (CRC), which is included with each packet sent over the packet switched network. In this conventional scheme, the node originating a packet generates an error detection code and appends the code to the packet for transmission. The receiving node (destination) then conventionally performs an error check on the received data packet to determine whether the packet suffered any transmission error. This fault detection scheme has the disadvantage of making error detection integral with the data transmission so that faults are detected only after data packets have been received at a destination node. This conventional scheme also has the added drawback that faults are merely detected and not isolated to a specific physical link. A conventional variation of this detection scheme performs a fault test on a packet at intermediate nodes in the network in order to isolate fault to a region within the network. This scheme, however, adds latency to the total transmission time of a packet and thus degrades network performance.
Another conventional fault detection scheme transmits a test packet from a test source along a node path in the network. Each node along the path transmits the test packet and awaits an acknowledgment from the receiving node. If no acknowledgment is received, the node reports an error along with error location information back along the node path to the test source. In this way, a fault is detected and isolated during a test mode of operation. Using this scheme, faults are detected and localized only during a test operating mode and requires the use of a dedicated test packet. This conventional fault detection scheme has the added disadvantage that in some cases, many test packets are required to test the entire network.
Thus, there is a need for a method and apparatus for efficiently detecting and localizing faults in a packet switched network that does not require the generation of dedicated test packets during a test mode and which detects faults even when data packets are not being sent.